It's basically a code telling itself to write more code, so the initial instructions don't need to be large at all.

Simple example:

1 Set "number" equal to 0

2 Set "number" equal to (number+1)

3 Save number in new file named "(number)"

4 Go to step 2

So in this entirely fake program, the idea is it will start with a variable =0. It will then change that variable to 1. Then it will make a save file called "1." Then it will change the variable to 2, and make a save file called "2." And it just keeps repeating, adding one more and then creating a new file in your computer. This would eventually take up every single bit of free memory and it will never stop running.